Different types of primary batteries are available. The most common types of primary batteries are Zn-carbon, alkali-manganese, and Li batteries. A special purposes type of cell is the metal-air cell of which the Zn-Air cell is the most common. Zn-Air button cells are commonly used for hearing aid applications. Another application for high power Zn-Air batteries is as a source of energy for emergency power supplies. Zinc-air cells have a very high energy density, e.g., up to 40% higher than the energy density of the most advanced Li-batteries. The high energy density results because the material for one electrode is inside the battery housing, which leads to the increased capacity for that material. The material for the second electrode is oxygen from ambient air.
While generating electric current the battery needs to receive continuously fresh air proportional to electrical power provided by the battery. Small button cells receive air by diffusion. For high power applications, the battery includes a fan or other mechanism to push air through the battery.
A major drawback for Zn-Air cells is that as soon as the cells come into contact with air, the cells start aging, even without delivering electrical power. This happens for two reasons. One reason is that the electrolyte, which is typically potash lye, is neutralized by carbon dioxide in the air. The second is that the cell dries out due to air exchange inside the battery.